The present invention relates to a lower extremity prosthetic fitting which is improved in respect of the transfer of loading forces between the fitting and the skeleton parts inside the shortened leg, the stump. Irrespective of whether the reason for the shortening was morbid changes of the lower portion of the leg which made it necessary to remove that portion by amputation, for instance due to diabetes, or the result of an accident, e.g. a traffic accident, the residual part of the lower extremity will assume the shape of a stump in which the skeleton parts are both laterally and at their lower ends surrounded by soft tissues. When, following healing after the shortening, a lower extremity prosthesis is to be fitted, this fitting is provided with a socket open at its top end and tapering in the downward direction. It is intended to surround the stump and at its lower, closed end connected to an artificial foot. The function of the socket is to maintain the fitting connected to the stump. In order for the fitting to fulfill that function the internal shape of the socket must in a special way match the shape of the stump so that the loading forces acting on the foot can be transferred to the stump and from there further on to the knee-joint, the thigh-bone and the hip-joint.
The first step in that force transfer, from the socket to the knee-joint, takes place via the residual part of the tibia. However, this transfer will be indirect in the sense that as mentioned above, this skeleton member is surrounded by soft tissues also at the bottom of the stump. These soft portions are deformed, especially during the transfer of the substantially vertically oriented forces which occur when the patient is standing, when he walks and, generally, when the distance between the artificial foot and the thigh is varying. In such situations there occurs a relative movement between the stump and the socket. This movement, in the vertical direction often referred to as "pumping" and in the transversal direction as "staggering", is undesired for two reasons. One reason is that the play results in instability and the second reason that it may, in combination with the pressure against the soft portions, develop infections and wounds, in the most serious cases to such an extent that the prosthesis must not be used during a longer or shorter period of time.
There does accordingly exist a need for a possibility, to the extent desired, to replace this indirect transfer of forces to the tibia, that is via the remaining soft portions of the leg below the knee-joint, by a rigid, mechanical transfer direct to the tibia, which would result in a corresponding elimination of the possibility of pumping and staggering including their detrimental consequences.
It is prior art in thigh-leg prostheses to attain such a stress-relief by a rigid force transfer from the prosthesis directly to the skeleton member in the thigh. In such prior art designs the connection to the thigh-bone has been created by means of a pin extending vertically upwards from the bottom of the socket and entering an axial bore from the bottom end of the thigh-bone. It is, however, for several reasons very difficult to use such a force transfer arrangement in low extremity prosthetic fittings. Those reasons are due to the anatomical differences, in terms of different thicknesses and different shapes, which exist between a thigh-bone and a tibia. In both those cases the bone tissue proper surrounds an axial extending space containing bone-marrow. While the thigh-bone, femur, exhibits a great transversal dimension--it is the thickest bone of the skeleton--and is almost circular in cross-section, the tibia is considerably more narrow and substantially triangular in cross-section. Further, its wall thickness is considerably less. Also in a grown up person it normally amounts to just 3-5 mms and, quite naturally, in children the tibia wall surrounding the central bone-marrow space is still thinner. In this context it must also be noted that the force-absorbing ability of a pin inserted axially in the bone-marrow space is limited as far as lateral forces are concerned. While it is true that such lateral forces are of a negligible magnitude when the amputee is standing still in an upright position, they increase to considerable magnitudes when he is walking in which situation also dynamic force components are added. Should the amputee for instance kick or slip, these forces become very high. For those reasons orthopaedists and prosthesis-technicians have had to establish that so far they have not succeeded to solve the pumping problem, its related inconveniences and the risks for the patients, by using in lower extremity prosthetic fittings the same type of mechanisms for transferring forces between the fitting and the skeleton members of the extremity as can be used in thigh-bone prostheses.